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If I try to fill the TreeView with the following code, then the text "myHouse" dissapears
on mouseUp
put "My House" into mydata[1]
put "Window" into mydata[1] [1]
set the ArrayData of widget "Tree_View_1" to myData
end mouseUp

#####
If I try to fill the TreeView with the following code, then the text "Window" dissapears

on mouseUp
put "Window" into mydata[1] [1]
put "My House" into mydata[1]
set the ArrayData of widget "Tree_View_1" to myData
end mouseUp

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If you do the same in the Code Insoector, the samme Error? occurs.

2. You simply overwrite the content of -> mydata{1} in both cases!
As you can see, even in your first example "My house" never appears in the the array and tree-view widget since it also got overwritten.

But the order of your commands is different (and important as we can see here), thus the result (disappearing "My House" string.

Best

Klaus

P.S.
Will mobe this thread to the beginner section, since there is definitively no relation to EXTERNALS or PLUGINS!